What is happening, and when

The HSE gives the clearest timeline of any national source: "Your pregnancy hormones cause normal breast tissue to change into milk-producing tissue. This change happens as early as the first trimester (weeks 1 to 12)."

Then the specific point people are looking for: "From about week 16, your breasts are ready to produce milk. This first milk is called colostrum and is yellow. It is often called 'liquid gold'."

And on leaking: "Your breasts may leak colostrum and you may be able to express small amounts by the end of week 36."

So the machinery is in place from mid-pregnancy, months before the baby arrives. Leaking is simply the visible evidence of that.

What it looks like

Colostrum is not what people expect milk to look like. It is typically:

Yellow or golden, though it can be clear, cream, or very pale
Thick and sticky rather than watery
Present in tiny amounts — drops, not a flow
Prone to drying to a crust on the nipple, which is often what people notice first rather than the leak itself

It may appear as a damp patch on a bra overnight, a stiffened patch on a top, or simply a bead at the nipple after a warm shower.

If you are not leaking

This is the more common worry, and it is misplaced. Plenty of people never leak a drop before birth and go on to feed without difficulty. Leaking before birth predicts nothing about milk supply afterwards. It is a plumbing observation, not a capacity test.

The other breast changes that arrive with it

The HSE lists what else to expect, which is useful because most of these get noticed at the same time and worried about separately:

A tingling sensation early on, with tenderness and soreness, which it attributes to rising progesterone.
Small bumps on the areola, called Montgomery's tubercles — sweat glands that produce an oily substance to moisturise the areola.
Nipples and areola darkening in colour as pregnancy progresses.
Veins on the surface of the breasts becoming more noticeable.
Stretch marks on the breasts.
Nipples and breasts becoming larger in the last few weeks.

ACOG groups the darkening with the other pigment changes of pregnancy, noting dark spots on the breasts, nipples or inner thighs are caused by an increase in the body's melanin. The NHS says the same darkening may persist slightly after birth: changes fade gradually, "although your nipples may remain a little darker".

Expressing colostrum before birth

This comes up constantly and the HSE's position is worth quoting exactly, because it contains a condition people skip: "Your breasts may leak colostrum and you may be able to express small amounts by the end of week 36. Only do this if advised to by your midwife or GP."

It gives the rationale for why it is offered at all: "Learning to hand express and store colostrum can help you improve your breast milk supply. It can make breastfeeding easier after your baby's birth."

Antenatal expressing is commonly suggested where a baby may need extra feeds soon after birth, for example with diabetes in pregnancy. It is not a routine task for everyone and it is not something to begin unprompted. Ask your midwife whether it applies to you rather than starting because the internet suggested it.

What it is for

Colostrum is not simply a small quantity of ordinary milk. It is a different substance, produced in tiny volumes because a newborn stomach is tiny, and it is dense in antibodies and protein rather than water. That is why the amount that appears in pregnancy looks alarmingly small to anyone expecting milk. A few millilitres is what a first feed is supposed to be, and the volume shifts within days of birth as mature milk comes in.

Managing the leaking

Breast pads in the bra, changed when damp. Washable or disposable both work.
Dark or patterned tops if patches are the concern.
A soft, well-fitting bra, ideally non-underwired late on, since breast shape changes considerably. Getting fitted in the third trimester rather than guessing saves a lot of discomfort.
Leave the crusts alone. Warm water in the shower loosens dried colostrum. Soap and scrubbing dry the skin and make cracking more likely.
Do not squeeze to check. Stimulating the nipple to see whether anything comes out is the most common way people convince themselves there is a problem.

What to report

The HSE names the breast changes that should be checked rather than assumed:

A lump on your breast, or a change in an existing lump. It notes that lumps in pregnancy are usually benign and caused by hormonal changes, and that breast cancer is not common in younger women or during pregnancy — but that lumps should still be checked by a GP before and after birth.
Bleeding from your nipples. The HSE says blood-stained fluid leaking from the nipples "can be normal, but it's important to see your GP or breastfeeding specialist if you notice it."
Damage or trauma to the breast or nipples.

Redness, heat and pain in one area of the breast, particularly with a temperature, is also worth reporting rather than waiting, since infection in the breast can occur before birth as well as after.

Everything else — yellow drops, damp patches, crusting, tenderness, visible veins, darker nipples, new bumps on the areola — is the ordinary business of a body preparing to feed a baby several months before the baby needs feeding.

Leaking Colostrum in Pregnancy